The objective of "Region VIII Mixing Zones and Dilution Policy" is to help states and Indian tribes upgrade methods for deriving water quality-based permit limits, improve the technical defensibility of National Pollutant Discharge Elimination System (NPDES) permits, and reduce risks associated with mixing zone and dilution practices. The basis for the policy was the desire of the region to clarify the approaches for using mixing zones in NPDES permits. Prior to the policy, some applications of mixing zones presumptively provided the entire stream flow for dilution, which often resulted in effluent plumes (with elevated pollutant concentrations) extending far downstream of the discharge. The clarifications sought to prevent applications of mixing zones that did not adequately control effluent plume size or quality, possibly posing considerable risk to sensitive downstream uses.

"EPA Region VIII Interim Guidance: Water Quality Standards for Indian Tribes" is intended to assist the Indian tribes in EPA Region 8 to qualify to administer water quality standards programs and to develop water quality standards pursuant to Clean Water Act (CWA) sections 518 and 303(c). The guidance includes recommendations on mixing zone policy development.

"EPA Region IV Guidance on Mixing Zones" provides a basis for Region 4 review and approval of state use of mixing zones in the development of effluent limitations. The guidance provides detailed descriptions of (1) appropriateness of assigning a mixing zone to a discharger, (2) the level of water quality that should be maintained in a mixing zone and surrounding waters, and (3) the factors governing the size and shape of a mixing zone.

"EPA Region IV Guidance on Mixing Zones" provides a basis for Region 4 review and approval of state use of mixing zones in the development of effluent limitations. The guidance provides detailed descriptions of (1) appropriateness of assigning a mixing zone to a discharger, (2) the level of water quality that should be maintained in a mixing zone and surrounding waters, and (3) the factors governing the size and shape of a mixing zone.

North Carolina Department of the Environment and Natural Resources. 1999. "Mixing Zones in North Carolina. (PDF)" North Carolina Department of the Environment and Natural Resources, Division of Water Quality. (28 K, 5 pp.)

Appendix C: Other Documents to Consider When Establishing Mixing Zones†

† Note: these documents are often referred to in mixing zone guidance, but they have been updated or superseded by newer guidance documents and rules.

FWPCA. 1968. "Water Quality Criteria" (Green Book). Federal Water Pollution Control Administration. Report of the National Technical Advisory Committee to the Secretary of the Interior. U.S. Department of Interior, Washington DC. [Superseded by Water Quality Standards Handbook, Second Edition (USEPA, 1994)]

NAS/NAE. 1973. "Water Quality Criteria 1972" (Blue Book). A Report of the Committee on Water Quality Criteria. National Academy of Science and National Academy of Engineers, Washington, DC. NTIS-PB 236199. USGPO #5501-00520. [Superseded by Water Quality Standards Handbook, Second Edition (USEPA, 1994)]

USEPA. 1995. Final Water Quality Guidance for the Great Lakes System; Final Rule. U.S. Environmental Protection Agency. "Federal Register," March 23, 1995, 60:15366. (Superseded by the 2000 Final Rule)